1. Field of the Invention
The present invention relates generally to sandwich panels and other related structural composite materials. Sandwich panels are typically made up of face sheets which are adhesively bonded to opposite sides of a core material to form the sandwich panel. In particular, the present invention relates to sandwich panels in which the face sheets are self-adhesive so that a separate adhesive film is not required for bonding to the core.
2. Description of Related Art
Sandwich panels are used in a wide variety of applications where high strength and light weight are required. The cores which are used in most sandwich panels are either lightweight honeycomb, rigid foam, paper or wood.
Honeycomb is a popular core material because of its high strength to weight ratio and resistance to fatigue failures. Honeycomb cores are made from a wide variety of products including metals and composite materials.
The face sheets which are bonded to each side of the sandwich core have also been made from a wide variety of materials which also include metals and composites. An important consideration in the formation of any sandwich panel is the adhesive which is used to bond the face sheets to the core. The adhesive must rigidly attach the facings to the core in order for loads to be transmitted from one facing to the other and to permit the structure to fulfill all the assumptions implied in the acceptance of the commonly used stress calculation methods. If the adhesive fails, the strength of the panel is severely compromised. The adhesive layer is especially critical in sandwich panels which use honeycomb cores because of the relatively small surface area over which the honeycomb edges contact the face sheets.
Honeycomb sandwich panels are used in many applications where stiffness and structural strength of the panel are primary considerations. Additionally, honeycomb sandwich panels are also widely used in the aerospace industry where the weight of the panel is also of primary importance. As a result, there has been and continues to be a concerted effort to reduce the weight of honeycomb sandwich panels without sacrificing structural strength. One area which has been investigated to reduce weight is the elimination of the separate adhesive layers. This has been accomplished by making the face sheets from composite materials which are self-adhesive. Exemplary self-adhesive face sheets are described in published European Patent Applications Nos. EP0927737 A1 and EP0819723 A1.
One procedure for applying face sheets to honeycomb involves forming a prepreg sheet which includes at least one fabric or fiber layer and an uncured prepreg resin. Prepreg is a term of art used in the composite materials industry to identify mat, fabric, nonwoven material or roving which has been preimpregnated with resin and which is ready for final curing. An adhesive is typically applied to the prepreg and it is then bonded to the honeycomb by curing of both the prepreg resin and adhesive resin at elevated temperature.
In those instances where the prepreg is bonded without using a separate adhesive (i.e., the prepreg is self-adhesive), the prepreg resin must meet the dual requirements of being suitable for preimpregnating the fiber layer while still providing adequate adhesion to the honeycomb. There is a present and continuing need to identify and develop prepreg resins which are suitable for use in self-adhesive prepregs to provide lightweight yet structurally strong sandwich panels.
In accordance with the present invention, prepreg face sheets have been developed that are useful as self-adhesive face sheets which are bonded to honeycomb to form sandwich panels that are lightweight, structurally strong and exhibit many other desirable properties. Sandwich panels incorporating the self-adhesive prepregs of the present invention exhibit high peel strength, good hot/wet properties, low solvent absorption, high resistance to core crushing and other properties that are desirable for aerospace applications.
Self-adhesive prepregs in accordance with the present invention include at least one fiber layer that is impregnated with a prepreg resin to form a prepreg which has a prepreg resin layer having a bonding surface which is bonded directly to the honeycomb during sandwich panel formation. The prepreg resin includes a thermoset resin, a curing agent and a sufficient amount of a viscosity control agent to provide a prepreg resin having a viscosity which is sufficient to allow the prepreg resin to be combined with the fiber layer to form the prepreg resin layer. As a feature of the present invention, it was discovered that certain types and sizes of thermoplastic particles may be used as fillet forming particles that can be incorporated into the prepreg resin in amounts sufficient to make the prepreg self-adhesive while not adversely affecting the viscosity or other properties of the resin which are required for its use as a prepreg resin.
As a further feature of the present invention, it was found that thermoplastic fillet forming particles, such as densified polyethersulfone, are not dissolved to any substantial degree when they are loaded into the prepreg resin. As a result, the prepreg resin can be loaded with enough fillet forming particles to substantially increase the bonding strength of the resin while at the same time not increasing the viscosity of the prepreg resin to unacceptable levels. It was discovered that the fillet forming particles dissolve during the curing process to provide the resin with flow characteristics that enhance fillet formation between the prepreg and honeycomb. Fillet size and shape are known to be an important consideration in the bonding of face sheets to honeycomb. In addition, the dissolved fillet forming particles provided toughening of the resin which improves overall bond strength.
The present invention involves not only the uncured self-adhesive prepregs, but also includes the prepregs after they have been attached to honeycomb and cured to form finished sandwich panels. The invention also covers methods for bonding the self-adhesive prepregs to honeycomb in order to form sandwich panels. The methods involve forming a self-adhesive prepreg by providing at least one fiber layer and a prepreg resin wherein the prepreg resin is combined with the fiber layer to form a prepreg resin layer having a bonding surface which is adapted to be bonded directly to the side surface of the honeycomb. The prepreg resin includes a thermosetting resin, such as epoxy cyanate, bismaleimide, and the like, a curing agent and a sufficient amount of a viscosity control agent so that the prepreg resin has a viscosity which is sufficient to allow the prepreg resin to be combined with the fiber layer to form the prepreg resin layer.
In accordance with the present invention, the step of forming a self-adhesive prepreg further includes the step of incorporating thermoplastic fillet forming particles into the prepreg resin in an amount sufficient to form a prepreg layer which is self-adhesive and wherein the fillet forming particles are not dissolved to any substantial degree in the prepreg resin. As a final step in the method, the self-adhesive prepreg is bonded to said honeycomb wherein the bonding involves curing the self-adhesive prepreg for a sufficient time and at a sufficient temperature to substantially dissolve the fillet forming particles.